In the manufacture of a wide variety of polymers it is common for them to contain impurities which are unwanted in final products made from the polymers. Such impurities typically include residual monomer, solvents that may have been used in the preparation of the polymer, and low molecular weight organic species such as dimers, trimers and other oligomers that may have been formed during the polymerization process.
An important area is the preparation of polymers and copolymers of styrene made by a continuous mass polymerization process, in which it is desirable to produce polymer products having a residual styrene monomer content below about 100 ppm and whose content of oligomeric species, e.g. styrene dimer, is also minimized. Such products are useful for the manufacture of food packaging, where migration of residual monomer from the polymer into the food can cause problems of taint, i.e. undesirable odor and/or flavor.
In such continuous mass polymerization processes the devolatilisation operation is frequently carried out in two sequential steps. In the first step the polymer mass containing up to 40% removable volatile material is preheated and fed into a vessel held at reduced pressure (usually called a flash tank). In this first devolatilisation the bulk of the volatile components is removed leaving a residual volatiles content in the polymer that in the case of polystyrene production is typically 500-1000 ppm styrene monomer. Various methods of delivering the preheated mass to the flash tank have been proposed in the existing art. In some cases the preheater takes the form of a shell and tube heat exchanger mounted vertically in the top of the flash tank.
In this arrangement preheated polymer mass exits from the preheater and falls to the base of the flash tank to form a pool of devolatilised polymer which is continuously removed by pumping. To increase the hold-up time of the preheated mass in the flash tank the preheated molten polymer mass may be fed into the flash tank through a horizontal distributor arrangement such as a pipe containing multiple peripheral apertures. These apertures may deliver the polymer mass onto trays or similar devices intended to further increase polymer hold-up and exposure to the reduced pressure environment within the flash tank. The vapors of the volatile components usually pass out from the top of the flash tank and are condensed in an external condenser.
GB-A-880,906 discloses a devolatiliser flash tank arrangement having a conical or dome-like upper surface which is externally cooled to condense and trap higher boiling components (dimers, trimers, etc) from the vapors removed during polystyrene devolatilisation. The objective of the arrangement disclosed in GB-A-880,906 is to obtain a stream of purified styrene vapor from the flash tank that can be condensed and re-used directly in the polymerization process. In one embodiment the preheated polymer mass can be delivered to the flash tank as an upward-flowing stream.
Following a first devolatilisation step in a styrenics mass polymerization process, the residual monomer content of the polymer can be further reduced by a second devolatilisation step in which a small amount of an inert volatile substance known as a stripping agent is admixed with the molten polymer mass. The resulting mixture is then subjected to reduced pressure in a flash tank devolatiliser. The stripping agent may for example consist of water, methanol or a solution of carbon dioxide in water. The stripping agent is admixed into the molten polymer by means such as a static mixer, which disperses the stripping agent finely throughout the polymer mass. Exposure of the polymer mass-stripping agent dispersion to reduced pressure subsequently creates bubbles within the mass. The resultant increase in mass surface area (foaming) enhances the rate of removal of residual monomer from the polymer. Typically 1-2 parts by weight of water per hundred parts of polymer are used as the stripping agent in such second-stage flash tank devolatilisation of polystyrene. The water is injected into the flowing molten polymer mass at the inlet of the static mixer and is typically dispersed therein as liquid droplets of about 5 micrometers diameter. At typical flash tank pressures of 10-15.times.10.sup.2 Pa these droplets can be envisaged as subsequently undergoing a hundredfold increase in diameter due to vaporization.
The molten polymer foam thus generated could theoretically have a specific volume of the order of 1-2 m.sup.3 /kg of polymer. For comparison the specific volume of the polymer per se would be 0.001 m.sup.3 /kg. Foam generation and mass volume expansion on such a scale could not be accommodated in any known practical devolatilisation equipment, and in fact such expansions are not reached.
An example of a known polymer devolatilisation apparatus using a static mixer is that disclosed by Craig in Advances in Polymer Technology, Volume 10, No. 4 (1990), Pages 323-325. The static mixer feeds to a distributor of a falling-strand flash tank.
There remains a need for an apparatus and method which provides the full beneficial devolatilisation effects of stripping agent expansion while avoiding the generation, especially in the flash tank, of undesirably large foam volumes of the polymer mass.